Summary
On the basis of statistical mechanics, it is shown that the following rules concerning the relative stability of different polymorphic modifications are usually obeyed. (1) The enthalpy of transition of two enantiotropic forms is always positive at temperatures above their transition point, and negative at temperatures below the transition point or between monotropic forms (heat-of-transition rule). (2) Instead of the heat of transition, the difference in the heats of fusion may generally be used (heat-of-fusion rule). (3) If a modification is less dense, it is less stable at absolute zero. (4) For hydrogen-bonded crystals, the modification for which the first absorption band in the infrared spectrum is at higher frequencies has the larger entropy. The possibility of exceptions is discussed.
Zusammenfassung
Mit Argumenten der statistischen Mechanik wird gezeigt, daß im Hinblick auf die relative Stabilität polymorpher Modifikationen von Molekülkristallen im allgemeinen folgende Regeln gelten. 1. Die Umwandlungsenthalpie zweier enantiotroper Formen ist über ihrem Umwandlungspunkt immer positiv, unter dem Umwandlungspunkt oder zwischen monotropen Formen negativ (Umwandlungswärme-Regel). 2. Statt der Umwandlungswärme kann meist auch die Differenz der Schmelzwärmen verwendet werden (Schmelzwärme-Regel). 3. Eine Modifikation von geringerer Dichte weist am absoluten Nullpunkt die geringere Stabilität auf. 4. Bilden zwei Modifikationen Kristalle mit Wasserstoffbrücken, so hat diejenige die größere Entropie, deren erste Bande im IR-Spektrum bei höheren Frequenzen liegt. Auf mögliche Ausnahmen von diesen Regeln wird hingewiesen.
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Burger, A., Ramberger, R. On the polymorphism of pharmaceuticals and other molecular crystals. I. Mikrochim Acta 72, 259–271 (1979). https://doi.org/10.1007/BF01197379
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DOI: https://doi.org/10.1007/BF01197379